A binder for an electrode active material in an aqueous electrolyte of a manganese dry cell, an alkali dry cell, a nickel-cadmium secondary battery, etc. includes starch, polyvinyl alcohol and carboxymethyl cellulose (see JP-A-1-175171, JP-A-1-105471, JP-A-51-5538, and JP-A-50-26500, the term "JP-A" as used herein means an "unexamined published Japanese patent application"); hydroxypropyl cellulose (see JP-A-63-24859 and JP-A-54-49541); regenerated cellulose (see JP-A-61-91872); polyvinyl chloride, polyvinyl pyrrolidone, and so forth. In a nonaqueous electrolyte of a lithium cell, etc., Teflon is generally used as a binder (see DENCHI Handbook, Denki Shoin (1980)).
In secondary batteries in which a positive or negative electrode repeatedly undergoes expansion and contraction on charging and discharging, the active material or conducting agent is apt to fall off to shorten the charge and discharge cycle life. In order to prevent this, proper selection of a binder is of particular importance. However, the above-mentioned conventional binders are unsatisfactory for preventing an active material and the like from falling off.
JP-A-3-108263 discloses a binder for a positive electrode for prevention of fall-off of an active material, making mention of an improvement in charge and discharge cycle life. However, the binder disclosed has poor affinity to an aqueous or nonaqueous electrolyte and attains a low utilization of the active material, and the charge and discharge cycle life reached is still insufficient. Besides an increase of the binder in amount is accompanied by a considerable rise in internal resistivity.
Use of a rubbery high polymer (i.e., an elastomer) as a binder for a negative electrode of a secondary battery has been proposed as described in U.S. Pat. No. 4,814,242 and JP-A-4-255670. However, any of the recommended binders exhibits poor adhesion to a collector, such as a metallic foil, so that the active material easily falls off.
Thus, a charge and discharge cycle life and a utilization of an active material (i.e., discharge capacity) conflict to each other, and it has been demanded to develop a binder by which both of these conflicting requirements may be met.
In a lithium battery as a primary battery, a spiral structure made of a sheet electrode has been adopted in order to reduce the internal resistivity. Some lithium batteries use a sheet of a metallic foil as a collector so as to cope with heavy load discharge. However, cases have been sometimes met with, in which a film of an active material coated on the collector with weak adhesion falls off the collector during assembly operation, such as sheet rolling, resulting in an increased rejection rate. From this viewpoint, a binder is ought to improve adhesion between an active material and a collector. For example, a binder for improving the adhesion is disclosed in JP-A-3-222258, but the performance properties are still insufficient.
On the other hand, with the recent tendency toward size reduction of electronic equipment, such as a portable phone, a personal computer and a video camera, secondary batteries of high energy density have been demanded. To meet the demand, nonaqueous secondary batteries have been extensively studied.
Negative electrode active materials for nonaqueous secondary batteries typically include metallic lithium and lithium alloys. The problem associated with these active materials is that metallic lithium dendrically grows during charging and discharging to cause an internal short circuit, involving a danger of ignition because of high activity of the dendrical metal per se. To solve the problem, a calcined carbonaceous material capable of intercalating and deintercalating lithium has recently been developed for practical use. Although this carbonaceous material is superior for its relatively reduced danger of ignition and high charge and discharge capacity, it has electrical conductivity per se so that metallic lithium is sometimes precipitated on the carbonaceous material at the time of overcharge or rapid charge. It eventually follows that lithium dendrically grows thereon. This problem has been dealt with by altering a charger or reducing the amount of the positive electrode active material to prevent overcharge. Where the latter solution is adopted, however, the limited amount of the active material leads to a limited discharge capacity. Further, the carbonaceous material has a relatively low density and therefore a low discharge capacity per unit volume. Thus, the discharge capacity is limited by both the amount of the active material and the capacity per unit volume.
In addition to metallic lithium, lithium alloys and the above-mentioned carbonaceous material, active materials so far proposed include the oxides for a negative electrode described in EP-A-567149 and the oxides for a positive electrode described in U.S. Pat. Nos. 4,302,518 and 4,507,371.